1. Field of the Invention
The present invention relates to a display device in which an oxide semiconductor is used and a method for manufacturing the same.
2. Description of the Related Art
A thin film transistor formed over a flat plate such as a glass substrate is manufactured using amorphous silicon or polycrystalline silicon, as typically seen in a liquid crystal display device. A thin film transistor manufactured using amorphous silicon has low field effect mobility, but such a transistor can be formed over a glass substrate with a larger area. On the other hand, a thin film transistor manufactured using a crystalline silicon has high field effect mobility, but due to a crystallization step such as laser annealing, such a transistor is not always suitable for being formed over a larger glass substrate.
In view of the foregoing, attention has been drawn to a technique by which a thin film transistor is manufactured using an oxide semiconductor and such a transistor is applied to an electronic device or an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique by which a thin film transistor is manufactured using zinc oxide (ZnO) or an In—Ga—Zn—O based oxide semiconductor as an oxide semiconductor film and such a transistor is used as a switching element or the like of an image display device.